[llvm-branch-commits] [BOLT][NFCI] Fix return type of BC::getSignedValueAtAddress (PR #91664)

[Amir Ayupov via llvm-branch-commits]

https://github.com/aaupov created https://github.com/llvm/llvm-project/pull/91664

getSignedValueAtAddress calls DataExtractor::getSigned which naturally
returns a signed value.

Test Plan: NFCI